In communication devices, such as Code Division Multiple Access (CDMA), Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMax), Wireless Local Area Network (WLAN) and Bluetooth or other Personal Area Networks (PAN) communication devices, the ability to amplify signals with a high peak-to-average ratio (PAR) is necessary to allow the transmission of the desired signal from a transmitter to a receiver device.
Communication devices may include a power amplifier (PA) to amplify the desired transmit signal to an energy level sufficient to allow propagation of the transmit signal to a desired receiver location. While ideal amplifiers provide constant gain for all input signals, known as linear operation, real amplifiers only perform linearly within certain practical limits. For example, in an ideal amplifier when the input signal applied to the amplifier is increased, the output resulting signal also increases by the same amount. However, in a real amplifier there is a point where the amplifier becomes saturated and cannot produce any more output power. This is known as clipping and results in distortion of the amplified output signal. Some amplifiers are designed to gradually reduce the gain as saturation is reached, thus resulting less excessive distortion of the output signal. This is known as amplifier compression. Either form of distortion will have a negative effect on the communication link between the transmitter and the receiver in the form of increased error rates.
Therefore, there is a need for a power amplifier which provides linear amplification of the desired high PAR transmit signal.
Another important aspect of a power amplifier is efficiency. Efficiency is the measure of how much of the input power is usefully applied to the amplifier's output. For example, class A amplifiers are very inefficient providing only 10-25% power efficiency. Class B amplifiers are much more efficient but suffer from high levels of crossover distortion. Class AB amplifiers can be used to avoid crossover distortion, but have relatively low efficiency varying from 35-78.5%. Class D amplifiers, also known as switching amplifiers, have efficiency as high as 97% and do not suffer from crossover distortion. An increase in amplifier efficiency results in a decrease in power consumption and heat generation. Therefore, in mobile communication devices where power is limited and heat dissipation is difficult, high efficiency amplifiers are highly desirable.
Non switching power amplifiers such as class-A or AB face inevitable trade-off between linearity and power efficiency. More often than not, efficiency must be sacrificed to meet adjacent channel leakage specification. In addition, because efficiency drops sharply at power back-off, the average efficiency when delivering high PAR signals is much less than peak efficiency. For example, for an ideal class-A power amplifier with 50% peak efficiency, efficiency is merely 7.4% for a clipped 802.11g signal with PAR of 8.3 dB.
Polar modulation is a popular choice to achieve better power efficiency than with a linear power amplifier. However, because amplifier delay is a function of supply voltage, AM-PM distortion compensation is usually necessary. The availability of fast PMOS transistors in advanced CMOS processes enabled class-D power amplifiers with delta-sigma modulated digital input to be used for RF applications. In contrast with linear power amplifiers, this type of power amplifier achieves high power efficiency and linearity simultaneously. Compared with a class-E amplifier, where a transistor drain has to sustain ˜3.5 times VDD of voltage swing, a class-D power amplifier does not have oxide issues and theoretically delivers the most power for a given oxide breakdown voltage.
Furthermore, because the amplitude modulation is performed in time domain, it is guaranteed to be linear and does not suffer the same AM-PM distortion of supply modulated polar power amplifier. However, the 1-bit quantization noise power must be shaped out-of-band, where they must be filtered by an external RF filter, which not only adds cost but also reduces power efficiency due to insertion loss. In the implementation described by J. T. Stauth and S. R. Sanders, “A 2.4 GHz, 20 dBm Class-D PA with Single-Bit Digital Polar Modulation in 90 nm CMOS,” IEEE Custom Integrated Circuits Conference, September 2008, even after the filter, out-of-band noise floor is still 30˜40 dB higher than a typical co-existence specification of WLAN and Bluetooth to enable simultaneous cellular/GPS receive operation.
FIG. 6 and FIG. 7 show a schematic diagram of two different conventional modulation schemes. FIG. 6 is a conventional digital envelope modulator for a WLAN OFDM polar transmitter described by P. van Zeijl in “A Digital Envelope modulator for WLAN OFDM Polar Transmitter in 90 nm CMOS” JSSC October, 2007. FIG. 7 is a conventional digitally modulated polar CMOS power amplifier described by A. Kavousian in “A Digitally Modulated Polar CMOS PA with 20 MHz Signal BW” ISSC, 2007. Both of these conventional schemes are essentially the same in that they both modulate amplitude with attenuation by resistance, therefore smaller amplitudes result in larger percentages of power loss. The resulting increase in power loss for smaller amplitudes results in sub-optimum power efficiency performance.
Suppression of quantization noise is another important aspect of a power amplifier. Quantization noise is the result of quantization error introduced by quantization in the analog-to-digital (ADC) process. Quantization error is the error between the analog input voltage to the ADC and the output digitized value. When the quantization error is modulated within the transmitter, quantization noise is created which results in out of band noise being amplified and transmitted. Out of band transmissions are undesirable because they may cause interference with other communication systems utilizing the out of band frequencies.
Therefore, there is also need for a power amplifier capable of reducing quantization noise.